However if the sat nav gives you a prior that says 35 mph, and the road sign says 85 mph, do you trust it? Especially if the adjacent roads are also low speed, and it's clearly not a highway etc. This is perhaps a bad example as there are plenty of roads in the UK that transition from national to 30 in rural areas (and plenty of hamlets that you can bomb through).

Roadworks are an obvious situation where you drive at a _lower_ speed. This would be a case where you could use computer vision to identity an obstruction, identify roadworks and then adjust speed.

Those databases shouldn't be out of date. Most highway agencies (UK for sure) publish planned works and improvements weeks in advance. Google and Apple both show road closures for example. I belive speed changes are similar.